1. Field of Invention
This invention relates generally to the field of multiple access digital communication, and more particularly, to methods, apparatus, media and signals for searching for a predefined code in a bit stream, as well as to systems incorporating same.
2. Description of Related Art
In the field of communications, there is often a need for multiple transmitters to transmit simultaneously on a shared transmission medium. In order to facilitate such shared access to a medium it is necessary to provide a method of avoiding interference between the transmissions of separate users. A number of methods for facilitating multiple access are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). Direct sequence spread spectrum (DS-SS) techniques, such as code division multiple access (CDMA) possess significant advantages over TDMA and FDMA however.
A direct sequence coding spread spectrum communication technique essentially encodes a digital data signal with a digital pseudo-random sequence (hereinafter, “pre-defined code”) of much higher bit rate to produce a third signal, a process sometimes called “spreading”. Individual bits of this pre-defined code are called chips.
This third signal may be modulated to a radio-frequency carrier and transmitted from an antenna. At a receiver, the radio-frequency carrier is demodulated to reproduce the third signal. The receiver generates a digital pseudo-random sequence of the same form as that of the received signal (hereinafter, “reference code”), and attempts to synchronize its phase to that of the received signal in order to recover the data thereof. DS-SS communication systems typically transmit such a code, also called a pilot signal, to allow users to synchronize to the transmitter and receive communications therefrom. However, multi-paths which are received separate instances of the pre-defined code at various amplitudes and phases due to atmospheric and environmental conditions, i.e., “echoes” of the pre-defined code may also be received.
Searching for a pre-defined code in a bit stream representing a received signal by attempting to correlate it with a reference code is often called “acquisition”. Generally, acquisition entails searching for the position (e.g., phase) of a pre-defined code in a received bit stream by attempting to correlate it with a plurality of reference codes, each generally of a form similar to the pre-defined code, but at a different position (e.g., phase) than the pre-defined code. In so doing, acquisition effectively tests multiple hypotheses of what the pre-defined code position (e.g., phase) might be in order to find a hypothesis of said position that is maximally likely for the given search parameters.
It will be appreciated that the overall search window may be very large, but the location of the pre-defined code will span a narrow region thereof (the “region of interest”), in which most of the energy from multi-paths of the pre-defined code subsists. For example, the overall search space for the pre-defined code may span thousands of chips, but most of the energy of multipaths of the pre-defined code may be confined within a narrow region of interest, spanning 13 microseconds or 16 chips, for example. Ideally, one would like to concentrate one's search in the region of interest, but without prior knowledge about what may be the region of interest such a search is not feasible.
It is known in the art that the accuracy of a search for the phase of the pre-defined code may be increased by performing a second search with a larger effective search time, known as dwell time. Such an approach is taken by dual dwell systems, which perform two searches. Typically dual-dwell systems attempt to locate a region of interest by a first coarse search at a first resolution, and then, once a sufficient correlation is found, they concentrate their search at the same resolution on the region of interest by increasing the dwell time over a smaller window spanning the region of interest, which yields a better estimate of the presence of a pilot signal in that region (i.e., a longer second dwell provides a reduction in the statistical variance of the energy estimation). The cost of this approach is that the second dwell must take a longer period in order to improve search accuracy. An approach reducing the time of the second dwell is needed.
It will further be appreciated that the received signal is analog and bandwidth-limited in nature. Thus, when the received analog signal is converted into a digital bit stream, the resultant signal will comprise pseudo-digital pulses which are only an approximation of the original digital bit stream in the transmitter before transmission. Various other radio-frequency signals, including multi-paths of the pilot signal itself, interfere with the received signal further. An acquisition method, means, code, apparatus, or system, may therefore find a correlation at a particular position or phase of the reference code, but this location or phase may be sub-optimal since it may be found at the edge of the bit of the pre-defined code in the incoming bit stream.
Unfortunately, a sub-optimal alignment of a receiver's reference code will lead to inaccuracies in despreading data in its received signal. Moreover, in a IS-95 CDMA system, an accurate determination of the phase of a pilot signal improves the timing of handoffs of cell phone communication channels between basestations. As such, there is a need for enhanced accuracy of pulse-targeting to ensure that the position or phase of a reference code may be accurately aligned to that of a pre-defined code in an incoming bit stream.